The range of uses of liquid crystal displays has been increasing because of many merits, such as possibility of direct connection to IC circuits at low voltage and at low power, a wide variety of display functions, high productivity and possibility of weight reduction.
Twisted nematic liquid displays wherein the twist angle of the liquid crystal molecules is 160 degrees or greater (hereinafter referred to as STN-LID) are practically used for dot matrix type liquid crystal displays used for office appliances such as word processors, personal computers, etc. at present and hold the first place. This is because a STN-LCD can keep high contrast even during high multiplex driving in comparison with conventional twisted nematic liquid displays wherein the twist angle is 90 degrees (TN-LCD). However, when a STN-LCD is used, it is impossible to whiten the color tone of the appearance of the displays, and the color tone is greenish to yellowish red, and hence the displays using STN are unsuitable for use as devices.
There has been proposed a method wherein one or more layers of optical anisotropic materials are provided between a pair of polarizing sheets to solve the problems mentioned above. When straight polarized light passed through one sheet of a pair of said polarizing sheets is passed through the liquid crystal layer of a liquid crystal element and the optical anisotropic materials, there is obtained elliptic polarized light nearly arranged in the direction of the major axis in the wavelength region of about 400 to about 700 nm. As a result, white light is obtained without causing the screening of light in a specific wavelength region when passed through the other sheet of the pair of said polarizing sheets.
There were patent applications filed relating to phase difference films for use in the removal of coloring of a STN-LCD. For example, there are disclosed in JP-A-63-189804 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") uniaxially stretched polycarbonate films having retardation values (a product of a birefringence and a film thickness) of 200 to 350 nm or 475 to 625 nm when measured with a polarization microscope. JP-A-63-167304 discloses film laminates obtained by laminating two or more uniaxially stretched films or sheets having birefringence properties so as to allow the optical major axes to fall at right angles with each other. When two sheets of double refractive films (retardation values being R.sub.1 and R.sub.2, respectively) are laminated onto each other so as to allow optical major axes to fall at right angles with each other, a phase difference film having a retardation of .vertline.R.sub.1 -R.sub.2 .vertline. as a laminate can be obtained. Accordingly, the aforesaid invention has an advantage in that a value of .vertline.R.sub.1 -R.sub.2 .vertline. can be controlled to a range of 90 to 180 nm, 200 to 350 nm or 475 to 625 nm even when R.sub.1 and R.sub.2 are large retardation values.
The above inventions are intended to solve the problem with regard to the coloring of STN-LCD. The aforesaid problem has been greatly improved and a display near monochromatic display has been obtained. Further, methods wherein a high-molecular birefringent film (hereinafter referred to as phase difference film) is used, have a merit in cost, and the film has come to be more in request.
The liquid crystal display using the phase difference film achieves color removal when seen from the direction vertical to the screen. However, when it is seen from an oblique direction, the disadvantage generally associated with a STN-LCD relating to viewing angle characteristics, i.e., coloring or disappearance of display with a slight change of viewing angle, becomes pronounced. This is a great problem to a STN-LCD.